This project is concerned with the characterization of synaptic relationships between identified axonal systems and the neuronal populations of the cat and monkey spinal cord and the somatosensory thalamus. The principal techniques to be used involve electron microscopy of axons labeled by degeneration, autoradiographic or intracellular horseradish peroxidase (HRP). Thalamic neurons will be stained by a modified Golgi method which permits description of the cells by light microscopy, and subsequent analysis of the same cell by electron microscopy. There are four principal aspects to the study: 1. Analysis of synaptic relationships of single physiologically characterized dorsal root axons subsequently injected with HRP. 2. Identification of descending projections to spinal neurons from brain stem utilizing axonal transport of radioactive proteins revealed by autoradiography. 3. A description of afferents to the monkey thalamus from dorsal column nuclei using degeneration and autoradiographic methods. 4. An analysis of thalamic circuity by combining degeneration and autoradiographic methods. The results of these studies will provide precise information concerning the neuronal networks subserving somatosensory information and descending systems believed to modify the activity of neurons mediating pain.